Rename tracking_id -> id, id -> class_id

msg/Detection2D.msg

@@ -16,6 +16,7 @@ BoundingBox2D bbox
 #   the image). Not required for all use cases, so it may be empty.
 sensor_msgs/Image source_img
 
-# ID used for consistency across multiple detection messages. This value will
-#   likely differ from the id field set in each individual ObjectHypothesis.
-string tracking_id
+# ID used for consistency across multiple detection messages. Detections
+# of the same object in different detection messages should have the same id.
+# This field may be empty.
+string id

msg/Detection3D.msg

@@ -18,6 +18,7 @@ BoundingBox3D bbox
 #   be empty.
 sensor_msgs/PointCloud2 source_cloud
 
-# ID used for consistency across multiple detection messages. This value will
-#   likely differ from the id field set in each individual ObjectHypothesis.
-string tracking_id
+# ID used for consistency across multiple detection messages. Detections
+# of the same object in different detection messages should have the same id.
+# This field may be empty.
+string id

msg/ObjectHypothesis.msg

@@ -5,7 +5,7 @@
 # The unique ID of the object class. To get additional information about
 #   this ID, such as its human-readable class name, listeners should perform a
 #   lookup in a metadata database. See vision_msgs/VisionInfo.msg for more detail.
-string id
+string class_id
 
 # The probability or confidence value of the detected object. By convention,
 #   this value should lie in the range [0-1].